Chief political correspondent

Australia's newest and most expensive defence technology acquisition, the high-tech F-35 Joint Strike Fighter, has been grounded in the US where it is being built, after one of the stealth bombers experienced an unexplained engine fire.

Australia is expected to take possession of its first two JSFs within weeks - more than two years after they were originally planned to be delivered.

The fire is the latest of a series of problems with the controversial single-engined aircraft which has suffered avionics and software issues and attracted harsh criticism from experts for being too slow, unmaneuverable, and likely to be superseded by the time it comes on line.

The US military, which is still in the pre-service stage as it assesses the capabilities and limitations of the aircraft, has decreed all F-35s must remain on the ground until the cause of the pre-take-off fire - in which the pilot was not injured - is identified.

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Australia has signed up for at least 58 of the Lockheed Martin planes to form the backbone of the country's air-defence force. Another squadron of 24 is also likely.

It is expected that the purchase bill for those will exceed $12.4 billion with maintenance costs likely to be even greater.

The Defence Minister, David Johnston, is expected to travel to the US shortly to receive the first of the planes officially.

A spokesman for Senator Johnston said the government was made aware of the fire "immediately", and said the engine difficulties would not alter Australia's acquisition timetable.

"The 'engine concerns' you refer to will not affect the purchase of 58 aircraft," the spokesman told Fairfax Media in response to questions.

He said the Pratt & Whitney-powered aircraft was performing well and would be highly suited to Australia's needs.

"To date the JSF aircraft has accrued 15,000 flight hours. While the F35 engine has successfully completed nearly 32,000 hours of testing, availability has remained steady at about 98 per cent.

"Single-engine fighters are operated by many air forces and Defence remains confident the F-35 JSF will be reliable and safe."

He added that Senator Johnston retained "full confidence in the JSF program" including in its handling of the current problem relating to the fire.

However, a source close to the program said the planes were too expensive and were beset with too many problems.

The source said the basic problem was the single engine which meant power failure automatically resulted in the loss of the $120 million plane if an engine broke down in flight.

Proponents of the F-35 say one of its greatest selling points is its claimed "inter-operability" which means its super-sophisticated computer and combat systems can "talk" to other planes and ground-based assets allowing for greater co-ordination.

But the source said it had never achieved these goals and was lagging behind in testing which explained why not one single F-35 was in service in the US.

According to Reuters news agency, the grounding has thrown into doubt plans for the F-35 to feature in two British air shows later this month.

In a prescient move, former defence minister Brendan Nelson overruled military advice during the last years of the Howard government to order FA18 Hornets to make up for a capability gap if the F-35 proved problematic.